In the production of color photographic images, it is usually necessary to remove the silver image which is formed coincident with the dye image. This can be done by oxidizing the silver by means of a suitable oxidizing agent, commonly referred to as a bleaching agent, in the presence of a halide ion, followed by dissolving the silver halide so formed in a silver halide solvent, commonly referred to as a fixing agent. Alternatively, the bleaching agent and fixing agent can be combined in a bleach fixing solution and the silver removed in one step by use of such solution.
In the reversal processing of black and white photographic materials, a bleaching step is also utilized to remove photographically developed silver.
A wide variety of bleaching agents are known for use in photographic processing. A particularly important class of bleaching agents are the aminopolycarboxylic acid bleaching agents. Such bleaching agents are described by K. H. Stephen and C. M. McDonald in Research Disclosure, Item 24023, April, 1984, and include the ferric complexes of ethylenediaminetetraacetic acid (EDTA) and propylenediaminetetraacetic (PDTA).
A particularly valuable bleach solution comprises the ferric complex of EDTA as the bleaching agent. However, due to advancements in emulsion technology and recent attempts to shorten photoprocessing times, there is a need to develop more powerful bleach solutions, that is, solutions that are more effective than EDTA bleaches at removing silver from photographic material. While a more powerful bleaching agent is desired, an improved bleach solution should not generate unacceptable stain or adversely affect process cleanliness or leuco cyan dye recovery.
Numerous bleaches have been developed and tested but have generally been found to be unsatisfactory for one or more of these reasons. One approach uses PDTA as a replacement for all or part of the EDTA. The improved effectiveness at removing silver from emulsions of PDTA bleaches compared to EDTA bleaches is known. The Research Disclosure noted above describes a bleaching solution including a mixture of the ferric complexes of EDTA and PDTA. However, the use of ferric complexes of PDTA heretofore had not been fully satisfactory, primarily because of the formation of ferric PDTA precipitates in processing solutions.
Thus, what has been desired is a bleaching solution which is better at removing silver from photographic materials than ferric EDTA bleaches and does not generate unacceptable stain or adversely affect leuco cyan dye recovery or process cleanliness.